Where the control is depends on whether you have a Girard Gen 2 or Gen 3 model. If your coach is new enough, you'll get the Gen 3. That will have a digital control mounted somewhere near the bathroom. It might look like a thermostat.
If you get a Gen 2, then the control is a dial (like a rheostat) in the bathroom. Turn it clockwise, toward "red" to make things hotter.
We've had two RVs and neither has had a strong shower by any sense of the word. The one time we had great flow -- we were hooked up to camground water (aka "City Fixtures") but, as I've posted, the Girard couldn't heat the water properly.
I hope it's better for you, but I'd set your expectations...